Transition metal and alloy nanoclusters are nanoscopic materials with significant value of applications, which can be used to develop various functional materials and devices (Y Wang, Y. G Wei, “Metal Nanoclusters” (chapter) in: H. S, Nalwa (Ed.), Encyclopedia of Nanoscience and Nanotechnology, Vol. 5, pp. 337-367, 2004, American Scientific Publishers). The inventors of the present invention had invented a kind of “unprotected” noble metal and alloy nanoclusters, as well as the method for preparing the same.
These metal nanoclusters, stabilized only with simple ions and organic solvent molecules, have small particle sizes and narrow size distributions, and can be produced in a large scale. Moreover, the “unprotected” metal nanoclusters can be conveniently separated as precipitates from the original dispersions and purified, which can be then re-dispersed into many kinds of organic solvents to form stable metal colloidal solutions (Y. Wang, J. W. Ren, K. Deng, L. L. Gui, Y. Q. Tang, Chem. Mater., 2000, 12, 1622; Chinese Patent, ZL 99100052.8). These metal nanoclusters have been used to synthesize catalysts (Y. Wang, et al., J. Catal., 2004, 222, 493), catalytic electrodes for fuel cells (S. Mao, G Mao, “Supported Nanoparticle Catalyst”, USA Patent, US 2003/0104936, A1, Jun. 5, 2003; Q. Xin, et al., App. Catal. B, 2003, 46, 273), and hydrogen sensors (Y. Wang, et al., Chem. Mater., 2002, 14, 3953), etc.
γ-Fe2O3 and Fe3O4 are two kinds of well known magnetic iron oxides, both of them have the cubic inverse spinel crystal structure. They can transform to each other in specific conditions. For example, the oxidation of Fe3O4 at about 523 K can produce γ-Fe2O3, indicating that γ-Fe2O3 is more stable than Fe3O4. Different from the conventional ferromagnetic iron oxides materials with large particle sizes, upon decreasing the particle size to some extent, the iron oxides nanoparticles may exhibit special electronic, magnetic and optical properties. These unique properties endow the iron oxides nanoparticles with extensive application value in the fields of ultrahigh density data storage, bio-separation, controllable release of medicine, and wave-absorption materials. Currently, the most common method for industrial production of γ-Fe2O3 is firstly preparing the ferric hydroxide precursor, followed by calcining the precursor at high temperature resulting in α-Fe2O3. Fe3O4 is then produced by the reduction of α-Fe2O3 with reductive gases, and then oxidized to γ-Fe2O3 at high temperature. The required temperature in the preparation process is usually higher than 523 K. using γ-Fe2O3 and Fe3O4 materials prepared in such a high-temperature method, it is difficult to fabricate nanoscopic materials with small sizes and excellent performances.
The combination of metal nanoclusters and oxide nanoparticles can endow the composite materials with various properties. Many effective methods have been developed for immobilizing metal nanoclusters onto support particles, such as the impregnation method, reduction-deposition method, adsorption of protected metal colloidal particles, coordination capture method, deposition method, and the encapsulation method, etc. Due to the different microstructures of the metal-inorganic oxide composite materials derived from dissimilar preparation methods, catalysts with the same chemical compositions may exhibit obviously different catalytic properties. The composition, structure, particle size and size distribution of nanocomposites can also significantly affect their catalytic properties.
The immobilization of protected metal colloidal particles (Y. Wang, et al., J. Chem. Soc. Chem. Commun, 1989, 1878) or the encapsulation technique (C. Lange, et al., Catal. Lett., 1999, 58, 207; A. Martino, et al., J. Catal., 1999, 187, 30; A. G Sault et al., J. Catal., 2000, 191, 474; H. Bönnemann, et al., Eur: J. Inorg. Chem, 2000, 5, 819; Top. Catal., 2002, 18, 265; G. A. Somorjai, et al., Chem. Mater., 2003, 15, 1242; J. Zhu, et al., Langmuir, 2003, 19, 4396) can be used to synthesize the metal-inorganic oxide nanocomposites. In the encapsulation technique, the inorganic oxides of alumina or silica, prepared by the in suit hydrolysis of the corresponding metal-alcohol salts [M(RO)n], are usually employed to encapsulate the metal colloidal particles protected by polymer, surfactant or coordination ligand. In order to obtain a close contact of the metal nanoparticles with inorganic supports, organic stabilizers originally adsorbed on the metal nanoclusters have to be removed by extraction or pyrolysis. This process may cause the aggregation of the metal nanoclusters, resulting in the difficulty in controlling the structure of the metal-inorganic oxide nanocomposites.
Seino et al. synthesized a kind of polyvinyl alcohol-metal-iron oxide magnetic nanocomposite materials by the photo-induced (using γ-irradiation) reduction of metal ions in aqueous solutions containing polyvinyl alcohol (PVA), and depositing the produced PVA-protected Au, Pt, Pd nanoclusters on commercially available γ-Fe2O3 particles with an average diameter of 26 nm or Fe3O4 particles with an average diameter of 100 nm (Scripta Materalia, 2004, 51, pp. 467-472). In this synthesis method, the concentration of iron oxides was relatively low (about 1 g/l), so the synthesis efficiency was not very high. On the other hand, the particle size of metal nanoclusters was dependent on the concentration of iron oxides particles. When Fe3O4 particles were used as the support, almost all of the metal particles deposited on the support were larger than 5 nm in size. In addition, the dispersion status of the oxide particles in the dispersion also affected the particle size of the deposited metal particles. Moreover, a part of PVA-protected Pt and Pd colloidal particles could not be adsorbed onto the iron oxides support.
Aromatic haloamines are important organic intermediates in the synthesis of dyes, pesticides, herbicides, medicines and special polymer materials. The hydrogenation of aromatic halonitro compounds to corresponding aromatic haloamines is one of important processes of chemical industry. It is a challenge in this synthesis industry to prevent the hydrogenolysis of the carbon-halogen bond in the haloaromatics, while maintaining the high catalytic activity of the catalysts for the hydrogenation of the aromatic halonitro compounds, especially when the conversion of the substrates is near 100%. If other electron-donating groups exist in the aromatic ring of the products, the hydrodehalogenation would become more serious (R. J. Maleski, et al., Eastman Chemical Co.) U.S. Pat. No. 6,034,276, (2000, 3, 7), WO 00/56698, 2000, 9, 2).
Over traditional metal catalysts (for example, Pt/C, Pd/C or Raney Ni), the hydrogenation of aromatic halonitro compounds was always companied by the hydrodehalogenation side reaction. Dehalogenation in the hydrogenation of bromine- or iodine-substituted aromatic nitro compounds is more serious than that in the case of aromatic chloronitro compounds. The order of susceptibility to hydrogenolysis for halogen-carbon bonds in aromatic halonitro compounds is I>Br>Cl>F (J. R. Kosak, in: Catalysis in Organic Syntheses, Academic Press, New York, 1980, pp. 107-117).
JP 2004277409-A (MITSUI CHEM. INC., JAPAN, 2004), disclose a technique for suppressing the hydrodechlorination of ortho-chloroaniline (o-CAN) over a Pt/C catalyst by charging 9.8 MPa of CO2 into the reaction system, which achieve a selectivity of 99.7 mol % to O-CAN. Obviously, this technology need highly expensive reactors, and could not completely suppress the dechlorination side reaction.
Over a Pt/TiO2 catalyst in a strong metal-support interaction state, the selective hydrogenation of para-chloronitrobenzene (p-CNB) was investigated under atmosphere pressure. When the conversion of p-CNB was less than 99.7%, the selectivity to para-chloroaniline (p-CAN) could reach 99.3%, which was the best selectivity in publications over Pt-based heterogeneous catalysts (B. Coq, A. Tijani, R. Dutartre, F. Figueras, J. Mol. Catal. A, 1993, 79, 253). However, after the complete conversion of the p-CNB substrate, the dechlorination rate of p-CAN increased rapidly. It is difficult to precisely control the reaction process in industrial production; thereby it is difficult to efficiently produce aromatic haloamines with high purity by using this catalyst.
Adding dechlorination inhibitors into the reaction system is also a method for suppressing the hydrodechlorination side reaction. EP473552-A (Baurneister, et al., 1992) described that in the hydrogenation of 2,4-dinitrochlorobenzene (2,4-DNCB) over a Pt/C catalyst modified with formamidine acetate, the selectivity to 4-chloro-m-phenylenediamine (4-CPDA) could reach 98% at complete conversion of the substrate.